A closed loop control system is the basic method employed to automate control of a system. One example of such a system is a servo control system which is a closed loop control of position and velocity. In order for such a system to work it must know where it is located at any given point in time. One method for achieving this requirement is by using a sensor to detect changes in movement. This can be accomplished by using different types of sensors. One such sensor is a linear voltage differential transducer (LVDT). An LVDT is a displacement transducer widely used to measure changes in the physical state of a system (i.e. fluid level, pressure, temperature, etc.). An LVDT has many applications such as sensing fluid levels, temperature, force, pressure, position and velocity among many others. LVDTs have gained wide use because their sensing elements have no actual physical contact when sensing. Therefore, there is very little wear on the LVDT while in use. A typical LVDT consists of a primary coil driven by an alternating current (AC) source, two secondary coils and a movable magnetic core. When the core is centered, the magnitude of the differential AC voltage output induced in the secondary coils will be the same in each coil. This state represents the zero or center point of the physical state being measured. When the core is displaced, the voltage across the secondary coils will change. This state represents a change or displacement in the physical state being measured.
In a typical application with instrumentation, the voltage output from the secondary coils of an LVDT must be rectified to a direct current (DC) signal. The DC signal is then typically processed by analog circuits to produce a voltage output which is linearly proportional to the displacement of the core and consequently the change in the physical state. This voltage output is then typically passed through an analog to digital converter in order to be used with today's computerized control systems.
Typical analog circuits used with LVDTs suffer from important limitations, however. They are subject to component drift and the output is temperature dependent. Some analog circuits use wiring known as ratiometric wiring which makes them more temperature independent. However, these require that the calculated sum of the input signals be constant to minimize errors. If the sum is not constant, high non-linearities in the processed signal can result. Additionally, current analog circuits only process signals for one dimension.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a decoder which will perform the function of the analog circuits used with LVDTs, but is not subject to the limitations of analog circuits used with LVDTs and can function in more than one dimension.